the bot up and down; there is no
counteracting force to balance it
out.

The easy solution is to have
teeth on both sides of the bar at
each location. Then as each tooth is
going up, it has a corresponding
one going down so the forces
balance each other out. I use a
variation of this on my bots where I
have two teeth on each side, but
with two of them closer together
and with the same center line
between each set. This allows each
tooth to get a better bite as it
contacts a target.

Make sure you get the design
right as its very hard — and perhaps
impossible — to correct this if you
get it wrong. If in doubt, ask
another builder.

Assuming you do get the
design right, then — in theory —
when you make the beater or drum
it should be balanced. However,
manufacturing tolerances mean that
it’s likely that it will still vibrate
badly. The reason for this is that the
weight on either side of the axle’s
center axis is not exactly the same.
The excess weight on one side
forces the bot up and down with
every rotation.

Up until now, I have simply
spun the beater on its axle to see
which side always ends up at the
bottom. I then remove a little
material evenly from that side and
spin it again until it no longer stops
consistently in one place.

This works pretty well but it’s
limited by the rolling friction in the
needle roller bearings commonly
used. Once the weight difference is
too small to overcome that friction,
this method no longer works.

However, I found the solution
one day while looking at the "New
Items" listing on the Hobby King
website ( www.hobbyking.com). It
came in the form of the Turnigy R/C
universal main blade balancer
(Figure1). It’s designed for
balancing props, but I saw that it
could be easily adapted to help
balance small beaters and drums.

FIGURE 2. Assembled.

It goes together easily with the
three rods tying together the two
carbon fiber side walls (Figure 2).
The ball bearing units are already
fitted for you. The device that
secures the prop can be discarded
because we won’t be using it.

The assembled balancer is just
the right size for a Weta beater bar
(Figure 3), and could be easily
modified to fit other sizes by
replacing the rods with longer ones.
The ball bearings are very free
running and allow much finer
balancing. The beater shown had
been balanced the old way, but
when placed on the ball bearings it
quickly became clear that it still was
out of balance and required a little
extra work.